The present invention relates to a combination of a cholesterol biosynthesis inhibitor and a .beta.-lactam cholesterol absorption inhibitor useful in reducing plasma cholesterol levels, and to a method of treating or preventing atherosclerosis comprising administering the claimed combination.
Plasma cholesterol levels have been positively correlated with the incidence of clinical events associated with coronary heart disease. The regulation of whole-body cholesterol homeostasis in humans and animals involves modulation of cholesterol biosynthesis, bile acid biosynthesis, and the catabolism of the cholesterol-containing plasma lipoproteins. The liver is the major organ responsible for cholesterol biosynthesis and catabolism and, for this reason, it is a prime determinant of plasma cholesterol levels. The liver is the site of synthesis and secretion of very low density lipoproteins (VLDL) which are subsequently metabolized to low density lipoproteins (LDL) in the circulation. LDL are the predominant cholesterol-carrying lipoproteins in the plasma and an increase in their concentration is correlated with increased atherosclerosis.
Another major factor in determining cholesterol homeostasis is the absorption of cholesterol in the small intestine. On a daily basis, the average human consuming a Western diet eats 300 to 500 mg of cholesterol. In addition, 600 to 1000 mg of cholesterol can traverse the intestines each day. This latter cholesterol is a component of bile and is secreted from the liver. The process of cholesterol absorption is complex and multifaceted. It has been reported that approximately 50% of the total cholesterol within the intestinal lumen is absorbed by the cells lining the intestines (i.e., enterocytes). This cholesterol includes both diet-derived and bile- or hepatic-derived cholesterol. Much of the newly-absorbed cholesterol in the enterocytes is esterified by the enzyme acyl-CoA:cholesterol acyltransferase (ACAT). Subsequently, these cholesteryl esters are packaged along with triglycerides and other components (i.e., phospholipids, apoproteins) into another lipoprotein class, chylomicrons.
Chylomicrons are secreted by intestinal cells into the lymph where they can then be transported to the blood. Virtually all of the cholesterol absorbed in the intestines is delivered to the liver by this route. When cholesterol absorption in the intestines is reduced, by whatever means, less cholesterol is delivered to the liver. The consequence of this action is a decreased hepatic lipoprotein (VLDL) production and an increase in the hepatic clearance of plasma cholesterol, mostly as LDL. Thus, the net effect of an inhibition of intestinal cholesterol absorption is a decrease in plasma cholesterol levels.
Beta-lactams such as (3R-4S)-1,4-bis-(4-methoxyphenyl)-3-(3-phenylpropyl)-2-azetidinone disclosed in PCT/US92/05972 are potent inhibitors of intestinal cholesterol absorption, leading to decreased plasma cholesterol levels in several animal species (hamsters, rats, rabbits, monkeys).
The inhibition of cholesterol biosynthesis by 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC1.1.1.34) inhibitors has been shown to be an effective way to reduce plasma cholesterol (Witzum, 1989) and reduce atheroscterosis. Combination therapy of an HMG CoA reductase inhibitor and a bile acid sequestrant has been demonstrated to be more effective in human hyperlipidemic patients than either agent in monotherapy (Illingworth, 1988).
We have unexpectedly found that a combination of a beta-lactam cholesterol absorption inhibitor and the HMG CoA reductase inhibitor lovastatin (MEVACOR.TM.) results in a greater decrease in plasma cholesterol than either agent alone in chow-fed dogs and rhesus monkeys, and in cholesterol-fed hamster and rabbits. These findings were unexpected because HMG CoA reductase inhibitors alone do not lower plasma cholesterol levels in hamster and monkeys.